US9568853B2 - Image forming apparatus including a plurality of driver IC configured to drive a plurality of light-emitting points - Google Patents

Image forming apparatus including a plurality of driver IC configured to drive a plurality of light-emitting points Download PDF

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US9568853B2
US9568853B2 US14/311,514 US201414311514A US9568853B2 US 9568853 B2 US9568853 B2 US 9568853B2 US 201414311514 A US201414311514 A US 201414311514A US 9568853 B2 US9568853 B2 US 9568853B2
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light
laser
power control
light power
emitting points
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US20140375744A1 (en
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Katsuyuki Yamazaki
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Canon Inc
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Canon Inc
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    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03G—ELECTROGRAPHY; ELECTROPHOTOGRAPHY; MAGNETOGRAPHY
    • G03G15/00—Apparatus for electrographic processes using a charge pattern
    • G03G15/04—Apparatus for electrographic processes using a charge pattern for exposing, i.e. imagewise exposure by optically projecting the original image on a photoconductive recording material
    • G03G15/043—Apparatus for electrographic processes using a charge pattern for exposing, i.e. imagewise exposure by optically projecting the original image on a photoconductive recording material with means for controlling illumination or exposure

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  • the present disclosure relates to a light power control method of a light source included in an electrophotographic image forming apparatus.
  • an electrophotographic image forming apparatus forms an image by exposing a photosensitive member to light with a plurality of rays of laser light to meet the demand of an increase in speed of image formation.
  • the electrophotographic image forming apparatus supplies bias current to each of a plurality of light-emitting points of the light source to ensure emission response of laser light (light beam) with which the photosensitive member is exposed to light. Since the plurality of light-emitting points each have a unique emission characteristic (relationship between current and light power), the value of bias current is set for each of the plurality of light-emitting points. Since the emission characteristic of each light-emitting point varies depending on the temperature of the light source, the image forming apparatus executes light power control on each light-emitting point during a period in which laser light does not scan on the photosensitive member when the image forming apparatus executes image formation on a recording medium.
  • PTL 1 discloses an image forming apparatus that executes first light power control (APC-H in PTL 1) and second light power control (APC-L in PTL 1) on each of a plurality of light-emitting points, and hence controls the value of bias current, which is supplied to each of the plurality of light-emitting points, based on the result of the first light power control and the result of the second light power control.
  • APC-H in PTL 1 first light power control
  • API-L second light power control
  • the image forming apparatus described in PTL 1 controls the value of current, which is supplied to each light-emitting point, in the first light power control so that the light power of laser light, which is emitted from each light-emitting point, becomes a first light power, and controls the value of current, which is supplied to each light-emitting point, in the second light power control immediately after the first light power control so that the light power of laser light, which is emitted from each light-emitting point, becomes a second light power.
  • the image forming apparatus of PTL 1 continuously executes the first light power control and the second light power control on a single light-emitting point, and then executes the first light power control and the second light power control similarly on another light-emitting point.
  • the image forming apparatus of PTL 1 executes the first light power control and the second light power control on all light-emitting points.
  • an image forming apparatus includes a light source including a plurality of light-emitting points that emit light beams for exposing a photosensitive member; a light-receiving unit configured to receive the plurality of light beams emitted from the plurality of light-emitting points; a light power control unit configured to execute first light power control controlling driving current supplied to each of the plurality of light-emitting points, so that a light power of the light beams received by the light-receiving unit becomes a first light power, and second light power control controlling driving current supplied to each of the plurality of light-emitting points, so that the light power of the light beams received by the light-receiving unit becomes a second light power; and a bias current control unit configured to control a value of bias current supplied to each of the plurality of light-emitting points based on results of the first light power control and the second light power control by the light power control unit.
  • the light power control unit is configured to execute the first light power control and the second light power control on the plurality of light-emitting points at different timings and continuously executes the first light power control on at least two or more light-emitting points among the plurality of light-emitting points.
  • FIG. 1 is a schematic cross-sectional view of an image forming apparatus according to this embodiment.
  • FIG. 2 is a schematic configuration diagram of an optical scanning device according to this embodiment.
  • FIG. 3A is an illustration showing arrangement of light-emitting points of a semiconductor laser.
  • FIG. 3B is an illustration showing exposure positions on a photosensitive drum.
  • FIG. 4 is a control block diagram of the image forming apparatus according to this embodiment.
  • FIG. 5 is an emission characteristic of a certain light-emitting point of the semiconductor laser.
  • FIG. 6 is a schematic configuration diagram of a laser driver.
  • FIGS. 7A and 7B are illustrations each showing a control mode.
  • FIGS. 8A to 8D are illustrations each explaining an execution order of light power control.
  • FIG. 9 is a timing chart of a scanning period.
  • FIGS. 10A to 10C are illustrations each showing a change with time of the output signal of a PD 204 in an APC sequence.
  • FIGS. 11A and 11B are illustrations each showing a change with time of the output signal of the PD 204 in an APC sequence of related art (comparative example).
  • FIG. 1 is a schematic cross-sectional view of the color image forming apparatus.
  • the image forming apparatus shown in FIG. 1 is a full-color printer that forms an image by using a plurality of colors of toners.
  • the full-color printer is described as an example of an image forming apparatus.
  • a monochrome printer that forms an image by using a monochrome toner (for example, black), or a color or monochrome copier including a reading device may be employed.
  • the image forming apparatus includes image forming units 101 Y, 101 M, 101 C, and 101 Bk that form images of respective colors.
  • the image forming units 101 Y, 101 M, 101 C, and 101 Bk form images by using respective toners of yellow (Y), magenta (M), cyan (C), and black (Bk).
  • the image forming units 101 Y, 101 M, 101 C, and 101 Bk respectively include photosensitive drums 102 Y, 102 M, 102 C, and 102 Bk each serving as a photosensitive member.
  • Charging devices 103 Y, 103 M, 103 C, and 103 Bk, optical scanning devices 104 Y, 104 M, 104 C, and 104 Bk, and developing devices 105 Y, 105 M, 105 C, and 105 Bk are respectively arranged around the photosensitive drums 102 Y, 102 M, 102 C, and 102 Bk.
  • drum cleaning devices 106 Y, 106 M, 106 C, and 106 Bk are respectively arranged around the photosensitive drums 102 Y, 102 M, 102 C, and 102 Bk.
  • An endless intermediate transfer belt 107 (intermediate transfer member) is arranged below the photosensitive drums 102 Y, 102 M, 102 C, and 102 Bk.
  • the intermediate transfer belt 107 is supported with tension by a driving roller 108 , a driven roller 109 , and a driven member 110 , and is rotationally driven in a direction indicated by arrow B in FIG. 1 during image formation.
  • primary transfer devices 111 Y, 111 M, 111 C, and 111 Bk are arranged at respective positions facing the photosensitive drums 102 Y, 102 M, 102 C, and 102 Bk through the intermediate transfer belt 107 .
  • the image forming apparatus 100 includes a secondary transfer device 112 that transfers toner images on the intermediate transfer belt 107 on a recording medium S, and a fixing device 113 that fixes the toner images on the recording medium S to the recording medium S.
  • Image forming processes of the image forming units 101 Y, 101 M, 101 C, and 101 Bk are the same. Hence, for example, the image forming process of the image forming unit 101 Y is described, and the description for the image forming processes of the image forming units 101 M, 101 C, and 101 Bk is omitted.
  • the surface of the photosensitive drum 102 Y which is rotationally driven in a rotation direction indicated by a solid-line arrow in FIG. 1 , is uniformly charged with electricity by the charging device 103 Y.
  • the charged photosensitive drum 102 Y is exposed to laser light LY (light beam) emitted from the optical scanning device 104 Y. Accordingly, an electrostatic latent image is formed on the photosensitive drum 102 Y. Then, the electrostatic latent image is developed by the developing device 105 Y, and becomes a yellow toner image.
  • laser light LY light beam
  • the primary transfer devices 111 Y, 111 M, 111 C, and 111 Bk apply a transfer bias to the intermediate transfer belt 107 . Accordingly, yellow, magenta, cyan, and black toner images on the photosensitive drums 102 Y, 102 M, 102 C, and 102 Bk are transferred on the intermediate transfer belt 107 . Consequently, a color toner image is formed on the intermediate transfer belt 107 .
  • the color toner image on the intermediate transfer belt 107 is transferred on a recording medium S by the secondary transfer device 112 , the recording medium S which is conveyed from a manual paper feed cassette 114 or a paper feed cassette 115 to a secondary transfer portion T 2 . Then, the color toner image on the recording medium S is heated and fixed by the fixing device 113 , and the recording medium S is ejected to a paper eject portion 116 .
  • Residual toners which are not transferred on the intermediate transfer belt 107 and remain on the photosensitive drums 102 Y, 102 M, 102 C, and 102 Bk, are respectively removed by the drum cleaning devices 106 Y, 106 M, 106 C, and 106 Bk. Then, the above-described image forming process is executed again.
  • FIG. 2 is a schematic configuration diagram of the optical scanning devices 104 Y, 104 M, 104 C, and 104 Bk.
  • the respective optical scanning devices have the same configuration, and hence FIG. 2 shows, for example, the optical scanning device 104 Y.
  • laser light which is emitted from a semiconductor laser 200 and is diverging, is collimated by a collimator lens 201 into substantially parallel light, and is shaped by limiting the passage of laser light by an aperture stop 202 .
  • the laser light which has passed through the aperture stop 202 , is incident on a beam splitter 203 .
  • the beam splitter 203 splits the laser light, which has passed through the aperture stop 202 , into laser light, which is incident on a photodiode 204 (light-receiving unit, hereinafter, PD 204 ), and laser light, which is directed to a rotatable polygonal mirror 205 (hereinafter, polygonal mirror 205 ) serving as a deflecting unit.
  • the PD 204 outputs a detection signal of a value (voltage) corresponding to the light power of laser light in response to the reception of the laser light.
  • the laser light which has passed through the beam splitter 203 , passes through a cylindrical lens 206 , and is incident on the polygonal mirror 205 .
  • the polygonal mirror 205 has a plurality of reflection surfaces (in this embodiment, four surfaces).
  • the polygonal mirror 205 is rotated in a direction indicated by arrow C when being driven by a motor 207 .
  • the polygonal mirror 205 deflects the laser light so that the laser light scans the photosensitive drum 102 Y in a direction indicated by arrow D.
  • the laser light deflected by the polygonal mirror 206 passes through an imaging optical system (f ⁇ lens) 208 , and is guided onto the photosensitive drum 102 Y (onto the photosensitive member) through a mirror 209 .
  • an imaging optical system f ⁇ lens
  • the optical scanning device 104 Y includes a beam detector 210 (hereinafter, BD 210 ) serving as a synchronization signal generating unit.
  • the BD 210 is arranged at a position on a scanning path of the laser light but outside an image formation region on the photosensitive drum 102 Y.
  • the BD 210 receives the laser light deflected by the polygonal mirror 205 and generates a horizontal synchronization signal.
  • FIG. 3A shows a plurality of light-emitting points included in the semiconductor laser 200 shown in FIG. 2 .
  • FIG. 3B is an illustration showing an arrangement image of laser spots on the photosensitive drum when laser light is simultaneously emitted from the plurality of light-emitting points.
  • the semiconductor laser 200 is a Vertical Cavity Surface Emitting Laser (VCSEL) including 32 light-emitting points 301 to 332 .
  • VCSEL Vertical Cavity Surface Emitting Laser
  • the semiconductor laser is not limited to VCSEL, and may employ an edge emitting laser.
  • the light-emitting points 301 to 332 are arranged in an array on a substrate 333 . Since the light-emitting points are arranged as shown in FIG. 3A , if the light-emitting points are simultaneously lit, laser light L 1 to laser light L 32 emitted from the respective light-emitting points expose different positions on the photosensitive drum in a main scanning direction to light like imaging positions S 1 to S 32 in FIG. 3B . Also, if the respective light-emitting points are simultaneously lit, the laser light L 1 to the laser light L 32 emitted from the respective light-emitting points expose different positions in a sub-scanning direction to light like the imaging positions S 1 to S 32 in FIG. 3B .
  • the arrangement of the plurality of light-emitting points may be two-dimensional arrangement.
  • FIG. 4 is a block diagram explaining an example of a control system used in the image forming apparatus shown in FIG. 1 .
  • the optical scanning devices also called laser scanners
  • 104 Y, 104 M, 104 C, and 104 Bk have the same configuration, and hence auxiliary characters Y, M, C, and Bk are omitted in the following description.
  • the configurations relating to 32 beams have parallel and repetitive configurations, and hence are partly omitted.
  • the image forming apparatus includes a CPU 401 , an image controller 402 , the optical scanning device 104 , the photosensitive drum 102 , a crystal oscillator 405 , a CPU bus 404 , and an EEPROM 410 .
  • the CPU 401 and the image controller 402 are included in an image forming apparatus body, and are connected with each optical scanning device 104 .
  • the optical scanning device 104 includes a first laser driver 405 A and a second laser driver 405 B.
  • the first laser driver 405 A, the second laser driver 405 B, and the light-emitting points 301 to 332 (light-emitting elements) corresponding to one color of Y, M, C, and Bk are described.
  • the first laser driver 405 A, the second laser driver 405 B, and the light-emitting points 301 to 332 are provided for each color of Y, M, C, and Bk.
  • the CPU 401 controls the entire image forming apparatus including the respective optical scanning devices 104 .
  • the CPU 401 receives supply of a reference clock with 100 MHz from the crystal oscillator 405 .
  • the CPU 401 generates 1 GHz by multiplying the reference clock by 10 by an embedded PLL circuit. This frequency is an image clock in a laser scanning system.
  • the image controller 402 separates image data received from an external information device connected with the image forming apparatus or the reading device attached to the image forming apparatus, into four-color components of Y, M, C, and Bk.
  • the image controller 402 outputs the image data of the four-color components of Y, M, C, and Bk to the CPU 401 through the CPU bus 404 in synchronization with the reference clock.
  • the CPU 401 stores the image data received from the image controller 402 , in a memory (not shown), and converts the image data stored in the memory into a differential signal (Low Differential Voltage Signal: LVDS) based on the image clock.
  • the CPU 401 outputs the differential signal to the laser drivers 405 A and 405 B at a timing based on a BD signal and an image clock signal.
  • the laser drivers 405 A and 405 B each generate a PWM signal based on the differential signal input from the CPU 401 , and emits laser light for forming an electrostatic latent image from the respective light-emitting points 301 to 332 based on the PWM signal. Also, the laser drivers 405 A and 405 B each execute Automatic light Power Control (APC) including first light power control, second light power control, and third light power control (described later), and hence control the light power of laser light for forming an electrostatic latent image, the value of bias current Ib serving as standby current, and the value of switching current Isw.
  • API Automatic light Power Control
  • the laser drivers 405 A and 405 B shown in FIG. 4 are each an IC of the same part model number, and each can control 16 light-emitting points.
  • the laser driver 405 A controls the light-emitting points 301 to 316
  • the laser driver 405 B controls the light-emitting points 317 to 332 .
  • a direct-current 5-V line and a ground line are supplied from a body rear-surface substrate (not shown). Electric power is supplied to the two laser drivers and the light-emitting points 301 to 332 from a common power supply.
  • the CPU 401 is connected with the laser drivers 405 A and 405 B by a plurality of signal lines as follows.
  • a signal line 406 A is a signal-line group that transmits a differential signal for driving the light-emitting points 301 to 316 from the CPU 401 to the laser driver 405 A.
  • a signal line 406 B is a signal-line group that transmits a differential signal for driving the light-emitting points 317 to 332 from the CPU 401 to the laser driver 405 B.
  • a signal line 407 A is a signal line that connects the CPU 401 with the laser driver 405 A.
  • a signal line 407 B is a signal line that connects the CPU 401 with the laser driver 405 B.
  • the CPU 401 transmits an IC select signal icsel_ 0 to the laser driver 405 A through the signal line 407 A, and transmits an IC select signal icsel_ 1 to the laser driver 405 B through the signal line 407 B. If the IC select signal icsel_ 0 is at H level, the IC select signal icsel_ 1 becomes at L level. If the IC select signal icsel_ 0 is at L level, the IC select signal icsel_ 1 becomes at H level.
  • the image forming apparatus of this embodiment executes APC on a light-emitting point that is controlled by a laser driver with an IC select signal to be input being at L level.
  • a signal line 408 and a signal line 409 are signal lines that connect the CPU 401 with the laser drivers 405 A and 405 B.
  • the signal lines 407 A, 407 B, 408 , and 409 are interfaces for transmitting control mode signals that set control modes of the laser drivers 405 A and 405 B (described later).
  • the laser drivers 405 A and 405 B execute various control based on the control mode signals transmitted from the CPU 401 .
  • the EEPROM 410 stores information relating to an APC sequence (described later).
  • the CPU 401 executes light power control on the light-emitting points in the order based on the information relating to the APC sequence stored in the EEPROM 410 .
  • DIS mode is set in an initial state immediately after the power supply of the image forming apparatus is turned ON. Also, DIS mode is set for interlock in a state in which a panel for maintenance is open for maintenance of the image forming apparatus. DIS mode is a state in which an electric charge is discharged from a hold capacitor (described later) and laser light is not emitted from a light-emitting point.
  • OFF mode is a mode set in a period (image non-formation period) other than a period (image formation period) in which laser light scans an image formation region on the photosensitive drum during image formation, and in a state in which a laser driver waits for an input of LVDS.
  • bias current Ib is supplied to each light-emitting point, however, switching current Isw is not supplied to each light-emitting point.
  • ACC mode of the image forming apparatus of this embodiment is a mode in which the light-emitting point 301 is forcedly lit so that laser light from the light-emitting point 301 scans the BD 210 in each scanning period.
  • VDO mode is a mode set in the image formation period. This is a mode in which bias current Ib is supplied to each light-emitting point, and switching current Isw is controlled to be turned ON/OFF based on a PWM signal generated from LVDS input to a laser driver.
  • APC mode is a mode in which APC is executed.
  • the value of bias current Ib is controlled based on the results of the first light power control and the second light power control in APC (described later).
  • the value of switching current Isw is controlled based on the result of the third light power control (described later).
  • the APC mode is a mode set in the image non-formation period to execute the first light power control, the second light power control, and the third light power control, in a period other than OFF mode.
  • FIG. 5 is an illustration showing an emission characteristic of a certain light-emitting point of the semiconductor laser.
  • the horizontal axis plots the current value supplied to the light-emitting point, and the vertical axis plots the light power of the laser light.
  • the curve in FIG. 5 indicates the light power of laser light with respect to the current value supplied to each light-emitting point.
  • the emission characteristic is a characteristic unique to each light-emitting point. Also, the emission characteristic is changed with temperature of the light-emitting point, and is changed with time.
  • the electrophotographic image forming apparatus is required to execute APC with high frequency to restrict generation of image density unevenness caused by variation in emission characteristic.
  • the increase in light power of laser light with respect to the increase amount of the current value is small in a region in which the value of current supplied to a light-emitting point is lower than a threshold current Ith, but in contrast, the increase amount of the light power of laser light with respect to the increase amount of the current is large in a region in which the value of current is higher than the threshold current Ith. If current with the threshold current Ith or lower is supplied, the semiconductor laser is not induced to oscillate, but spontaneously emits light. Since the light power produced by the spontaneous emission is very small, even if the spontaneous emission is provided, the potential of the photosensitive drum is not changed.
  • the bias current Ib having a value near the threshold current Ith is supplied to a light-emitting point to restrict a decrease in emission response.
  • the bias current Ib by supplying the switching current Isw based on the PWM signal generated from LVDS, laser light with intensity that changes the potential of the surface of the photosensitive drum is emitted from the light-emitting point.
  • the reach time of the laser light to the target light power can be decreased as compared with a case in which the light-emitting point is lit from a state in which the bias current Ib is not supplied.
  • the laser driver 405 A and the laser driver 405 B execute the first light power control and the second light power control at different timings on the light-emitting points 301 to 332 .
  • the first light power control and the second light power control are described by using the laser driver 405 A and the light-emitting point 301 .
  • the laser driver 405 A executes the first light power control that controls the value of current supplied to the light-emitting point 301 so that the light power received by the PD 204 becomes Pm.
  • the laser driver 405 A holds a current value Im corresponding to the light power Pm as the control result of the first light power control.
  • the laser driver 405 A holds a current value Il corresponding to the light power Pl as the control result of the second light power control.
  • the laser driver 405 A executes the first light power control and the second light power control on the light-emitting point 301 , the laser driver 405 A supplies only the bias current Ib with a value corresponding to each of the light-emitting points 302 to 316 , to the light-emitting points 302 to 316 . Also, the laser driver 405 B similarly supplies only the bias current Ib corresponding to each of the light-emitting points 317 to 332 , to the light-emitting points 317 to 332 (OFF mode).
  • the laser driver 405 A obtains the intersection of the segment connecting (Im, Pm) and (Il, Pl) (correspondence) and the axis indicative of the light power being “0” in FIG. 5 by calculation, and sets the value of the intersection at the current threshold Ith.
  • the laser driver 405 A updates (resets) the value of bias current Ib by multiplying the current threshold Ith by a predetermined coefficient ⁇ .
  • the coefficient ⁇ is previously set in accordance with the sensitivity of the sensitive drum attached to the image forming apparatus, and may be a value being 1 or larger, or a value smaller than 1.
  • the laser driver 405 A holds a current value Ih corresponding to the light power Ph as the control result of the third light power control.
  • FIG. 6 is an illustration showing an inner configuration of the laser driver 405 A.
  • the laser driver 405 B has the same inner configuration as the inner configuration of the laser driver 405 A. Hence, the description of the laser driver 405 B is omitted.
  • the laser driver 405 A includes a mode channel decoder 633 . Also, the laser driver 405 A includes driving units 617 to 632 , LVDS receivers 601 to 616 , AND circuits 652 , OR circuits 643 , transistors 644 , and switching power supplies 645 , respectively corresponding to the light-emitting points 301 to 316 .
  • the laser driver 405 A includes a first voltage output unit 636 that outputs a target voltage Vm (comparison signal) corresponding to the first light power (Pm) to the light-emitting points 301 to 316 , a second voltage output unit 637 that outputs a target voltage Vl (comparison signal) corresponding to the second light power (Pl) to the light-emitting points 301 to 316 , and a third voltage output unit 638 that outputs a target voltage Vh (comparison signal) corresponding to the third light power Ph to the light-emitting points 301 to 316 .
  • the laser driver 405 A includes a selector 640 , a comparator 641 , an EVR 642 , the mode channel decoder 633 , a selector 634 , and a resistor 635 .
  • the mode channel decoder 633 has a function of changing the control mode of the laser driver 405 A among DIS mode, VDO mode, OFF mode, ACC mode, and APC mode, based on a mode select signal, a channel select signal, and an IC select signal from the CPU 401 .
  • the CPU 401 outputs the IC select signal (icsel_ 0 ) to the mode channel decoder 633 .
  • the mode channel decoder 633 controls the laser driver 405 A to be in the APC mode based on the IC select signal from the CPU 401 .
  • the mode channel decoder provided in the laser driver 405 B controls the laser driver 405 B to be in the APC mode based on the IC select signal from the CPU 401 if the laser driver 405 A is not in the APC mode at a timing at which APC should be executed. That is, one of the laser driver 405 A and the laser driver 405 B is selectively transitioned to the APC mode by the IC select signal at the timing at which APC is executed.
  • the CPU 401 outputs a mode select signal group (ms 0 , ms 1 , ms 2 , ms 3 ) and a channel select signal group (ch 0 , ch 1 , ch 2 , ch 3 ) to the mode channel decoder 633 .
  • the mode channel decoder 633 generates APC mode signals (APCH_ON 1 - 16 , APCM_ON 1 - 16 , APCL_ON 1 - 16 ) based on the mode select signal group and the channel select signal group from the CPU 401 .
  • the mode channel decoder 633 outputs an APC mode signal to the laser driver 405 A in the APC mode.
  • the APC mode signal APCH_ON is a signal that causes the laser driver 405 A to execute the third light power control.
  • the APC mode signal APCM_ON is a signal that causes the laser driver 405 A to execute the first light power control.
  • the APC mode signal APCL_ON is a signal that causes the laser driver 405 A to execute the second light power control.
  • the mode channel decoder 633 outputs the APC mode signals APCH_ON, APCM_ON, and APCL_ON to each of the light-emitting points 301 to 316 at different timings. That is, the mode channel decoder 633 generates 48 APC mode signals in total including the APC mode signals APCH_ON 1 - 16 , the APC mode signals APCM_ON 1 - 16 , and the APC mode signals APCL_ON 1 - 16 . One signal among the 48 APC mode signals is at H level.
  • the laser drivers 405 A and 405 B each execute the light power control on the light-emitting point corresponding to the APC mode signal output from the mode channel decoder 633 , which is included in each of the laser drivers 405 A and 405 B.
  • FIG. 7A is a table showing the mode select signals, the channel select signals, and the IC select signal output from the CPU and corresponding to the various control modes.
  • DIS represents DIS mode
  • ACC represents ACC mode
  • VDO represents VDO mode
  • OFF represents OFF mode
  • APCH “APCM,” and “APCL” respectively represent the third light power control, the first light power control, and the second light power control.
  • Wording “ic” represents IC select signals icsel_ 0 and icsel_ 1 . If the input mode select signal indicates execution of APC and the IC select signal is at L level, the laser drivers 405 A and 405 B are brought into a state in which the laser drivers 405 A and 405 B can execute the first light power control, the second light power control, and the third light power control.
  • Each control mode is controlled based on a combination of the mode select signals ms 0 , ms 1 , ms 2 , and ms 3 shown in FIG. 7A .
  • [1] in the table indicates all combinations other than a combination of mode select signals in any of DIS mode, ACC mode, APCH mode, APCM mode, and APCL mode.
  • [2] in the table represents that the control state is determined regardless of the IC select signal or the channel select signal (ch 0 , ch 1 , ch 2 , ch 3 ).
  • [*] in the table indicates a combination of channel select signals shown in FIG. 7B . Characters e 1 to e 16 in FIG. 7B respectively correspond to the light-emitting points 301 to 316 .
  • the first light power control is executed on the light-emitting point 305 .
  • the mode channel decoder 633 controls only APCM_ON 5 at H level among the 48 APC mode signals, based on the mode-select signals and the channel select signals, and controls the other APC mode signals at L level.
  • the driving units 617 to 632 are described.
  • the driving units 617 to 632 are provided respectively for the light-emitting points 301 to 316 , and each supply driving current to the corresponding light-emitting point.
  • the driving units 617 to 632 have the same configuration, and hence an inner configuration of the driving unit 617 is exemplarily described.
  • the driving unit 617 includes an M hold capacitor 647 , an L hold capacitor 648 , an Ib calculation unit 649 , the selector 634 , and a bias current source 651 . Also, the driving unit 617 includes the AND circuit 652 , the OR circuit 643 , a transistor 644 , a switching current source 645 , an H hold capacitor 646 , and a voltage regulating circuit 653 .
  • the bias current source 651 and the switching current source 645 are connected with the light-emitting point 301 .
  • the bias current source 651 and the switching current source 645 are lead-in current sources that respectively lead the bias current Ib and the switching current Isw from VCC.
  • the bias current Ib is supplied to the light-emitting point 301 by the bias current source 651 .
  • the Ib calculation unit 649 is connected with the M hold capacitor 647 and the L hold capacitor 648 .
  • the Ib calculation unit 649 calculates the value of bias current Ib based on the control result of the first light power control (voltage of M hold capacitor 647 ) and the control result of the second light power control (voltage of L hold capacitor 648 ).
  • the LVDS receiver 601 receives a differential signal, which is image data, from the CPU 401 .
  • the LVDS receiver 601 outputs a PWM signal to the AND circuit 652 based on the differential signal.
  • the PWM signal is input from the LVDS receiver 601 to one terminal of the AND circuit 652
  • a VDO mode signal is input from the mode channel decoder 633 to the other terminal of the AND circuit 652 .
  • the AND circuit 652 If the VDO mode signal input to the AND circuit 652 is at H level and if the PWM signal is at H level, the AND circuit 652 outputs a signal at H level. If at least one of the VDO mode signal and the PWM signal input to the AND circuit 652 is at H level, the AND circuit 652 outputs a signal at L level.
  • the output signal from the AND circuit 652 is input to one terminal of the OR circuit 643 , and APCH_ON 1 , which is an APC mode signal from the mode channel decoder 633 , is input to the other terminal of the OR circuit. If at least one of the output signal from the AND circuit 652 and APCH_ON 1 is at H level, the OR circuit 643 outputs a signal at H level. If both the output signal from the AND circuit 652 and APCH_ON 1 are at L level, the OR circuit 643 outputs a signal at L level.
  • the output of the OR circuit 643 is connected with a base terminal of the transistor 644 .
  • a collector terminal of the transistor 644 is connected with the light-emitting point 301 .
  • an emitter terminal of the transistor 644 is connected with the switching current source 645 . If the signal at H level is output from the OR circuit 643 , the switching current source 645 leads the switching current Isw from VCC. Accordingly, the switching current Isw is supplied to the light-emitting point 301 for emitting laser light. If the signal at L level is output from the OR circuit 643 , the area between the collector terminal and the emitter terminal of the transistor 644 becomes a current non-conducting state.
  • the selector 640 selects one of the output signal (Vh) of the APCH target voltage output unit 636 , the output signal (Vm) of the APCH target voltage output unit 637 , and the output signal (Vl) of the APCH target voltage output unit 638 , based on APCH_ON 1 - 16 , APCM_ON 1 - 16 , and APCL_ON 1 - 16 output from the mode channel decoder 633 .
  • the output signal Vh from the APCH target voltage output unit 636 is a voltage corresponding to the third light power Ph (target light power).
  • the output signal Vm from the APCM target voltage output unit 637 is a voltage corresponding to the first light power Pm (target light power).
  • the output signal Vl from the APCL target voltage output unit 638 is a voltage corresponding to the second light power Pl (target light power).
  • the selector 634 includes a terminal 634 com that is connected with the comparator 641 , a terminal 634 gnd that is grounded, and terminals 634 - 1 to 634 - 48 .
  • the terminal 634 - 1 is connected with the H hold capacitor 646 of the driving unit 617 .
  • the terminal 634 - 2 is connected with the M hold capacitor 647 of the driving unit 617 .
  • the terminal 634 - 3 is connected with the L hold capacitor 648 of the driving unit 617 .
  • the other terminals 634 - 4 to 634 - 48 are similarly connected with the corresponding driving units.
  • the selector 634 receives the APC mode signals APCH_ON 1 - 16 , APCM_ON 1 - 16 , APCL_ON 1 - 16 , the OFF mode signal, the VDO mode signal, and the ACC mode signal from the mode channel decoder 633 . If the VDO mode signal, the OFF mode signal, and the ACC mode signal are input, the selector 634 connects the terminal 634 com with the terminal 634 gnd so that charging or discharging of the H hold capacitor 646 , the M hold capacitor 647 , or the L hold capacitor 648 is not performed.
  • a selector 650 provided in the driving unit 617 receives the APC mode signals APCH_ON 1 , APCM_ON 1 , APCL_ON 1 , the VDO mode signal, the OFF mode signal, and the ACC mode signal from the mode channel decoder 633 .
  • the driving units 618 to 632 receive corresponding APC mode signals.
  • the selector 650 includes a terminal 650 - 1 connected with the M hold capacitor 647 , a terminal 650 - 2 connected with the Ib calculation unit 649 , a terminal 650 - 3 connected with the L hold capacitor 648 , and a terminal 650 - 4 connected with the bias current source 651 .
  • the selector 650 connects the terminal 650 - 2 with the terminal 650 - 4 . If APCM_ON 1 is input, the selector 650 connects the terminal 650 - 1 with the terminal 650 - 4 . If the APCL_ON 1 is input, the selector 650 connects the terminal 650 - 3 with the terminal 650 - 4 .
  • the EVR 642 receives a detection signal from the PD 204 .
  • the EVR 642 has a function of correcting the detection signal to a value corresponding to each light source based on the light power adjustment table.
  • An EVR 642 receives an input.
  • the EVR 642 receives the APCH_ON 1 - 16 , APCM_ON 1 - 16 , and APCL_ON 1 - 16 .
  • magnification adjustment coefficients corresponding to optical collection efficiencies of the PD sensor and respective laser elements previously measured in a factory and set in the resistor 635 in an APC preparation phase are prepared as table data, and a table is selected in accordance with APCH_ON 1 - 16 , APCM_ON 1 - 16 , and APCL_ON 1 - 16 .
  • the CPU 401 executes the first light power control that controls the voltage of the M hold capacitor 647 .
  • the mode channel decoder 633 outputs the APC mode signal APCM_ON 1 for execution of the first light power control on the light-emitting point 301 to the selector 634 , the selector 640 , and the selector 650 based on the mode select signal and the channel select signal from the CPU 401 .
  • the selector 634 connects the terminal 634 com with the terminal 634 - 2 in response to the input of the APC mode signal APCM_ON 1 .
  • the selector 640 selects a comparison signal Stm output from the target voltage output unit 637 in response to the input of the APC mode signal APCM_ON 1 , and inputs the comparison signal Stm to the comparator 641 .
  • the selector 650 connects the terminal 650 - 1 with the terminal 650 - 4 in response to the input of the APC mode signal APCM_ON 1 .
  • the bias current source 651 leads current with a value based on the voltage of the M hold capacitor 647 from VCC. With this current, the light-emitting point 301 emits laser light. The laser light emitted from the light-emitting point 301 is incident on the PD 204 . The PD 204 outputs a detection signal corresponding to the light power of the laser light.
  • the comparator 641 compares the comparison signal Vm from the selector 640 with an amplification signal Samp from the amplifier circuit 642 , and outputs the signal based on the comparison result to the selector 634 .
  • Vamp voltage of Samp
  • the comparator 641 causes the M hold capacitor 647 to discharge electricity. If discharge of the M hold capacitor 647 is continued, the light power of laser light incident on the PD 204 is decreased, and becomes close to the first light power Pm.
  • Vamp Vm
  • the comparator 641 holds the voltage of the M hold capacitor 647 in this state.
  • the light power of laser light emitted from the light-emitting point 301 and being incident on the PD 204 is controlled to be the first light power.
  • the CPU 401 executes the second light power control that controls the voltage of the L hold capacitor 648 .
  • the mode channel decoder 633 outputs the APC mode signal APCL_ON 1 for execution of the second light power control on the light-emitting point 301 to the selector 634 , the selector 640 , and the selector 650 based on the mode select signal from the CPU 401 .
  • the selector 634 connects the terminal 634 com with the terminal 634 - 3 in response to the input of the APC mode signal APCL_ON 1 .
  • the selector 640 selects the comparison signal Vl output from the target voltage output unit 638 in response to the input of the APC mode signal APCL_ON 1 , and inputs the comparison signal Vl to the comparator 641 .
  • the selector 650 connects the terminal 650 - 3 with the terminal 650 - 4 in response to the input of the APC mode signal APCL_ON 1 .
  • the bias current source 651 leads current with a value based on the voltage of the L hold capacitor 648 from VCC. With this current, the light-emitting point 301 emits laser light. The laser light emitted from the light-emitting point 301 is incident on the PD 204 . The PD 204 outputs a detection signal corresponding to the light power of the laser light.
  • the comparator 641 compares the comparison signal Vl from the selector 640 with the amplification signal Samp (Vamp) from the amplifier circuit 642 , and outputs the signal based on the comparison result to the selector 634 .
  • Vamp amplification signal
  • the comparator 641 causes the L hold capacitor 648 to discharge electricity. If discharge of the L hold capacitor 648 is continued, the light power of laser light incident on the PD 204 is decreased, and becomes close to the second light power Pl.
  • the comparator 641 holds the voltage of the L hold capacitor 648 in this state.
  • the second light power control of APC by controlling the voltage of the L hold capacitor 648 , the light power of laser light emitted from the light-emitting point 301 and being incident on the PD 204 is controlled to be the second light power Pl.
  • the Ib calculation unit 649 which is a bias current control unit, calculates the value of bias current Ib based on the control result of the first light power control and the control result of the second light power control.
  • the calculation method is as described above.
  • the selector 650 connects the terminal 650 - 2 with the terminal 650 - 4 .
  • the Ib calculation unit 649 calculates the value of bias current Ib, and outputs a control signal, which is the calculation result, to the bias current source 651 .
  • the bias current source 651 leads the bias current with the value based on the control signal from the Ib calculation unit 649 from VCC.
  • the value of bias current is similarly controlled for any of the other light-emitting points 302 to 332 .
  • the value of switching current Isw is determined depending on the voltage of the H hold capacitor 646 .
  • the CPU 401 executes the third light power control that controls the voltage of the H hold capacitor 646 to control the value of switching current Isw.
  • the third light power control for the light-emitting point 301 is executed in a state in which the bias current Ib is supplied to the light-emitting point 301 .
  • the CPU 401 executes the third light power control that controls the voltage of the M hold capacitor 647 .
  • the mode channel decoder 633 outputs the APC mode signal APCH_ON 1 for execution of the third light power control on the light-emitting point 301 to the selector 634 , the selector 640 , the selector 650 , and the OR circuit 643 based on the mode select signal from the CPU 401 .
  • the selector 634 connects the terminal 634 com with the terminal 634 - 1 in response to the input of the APC mode signal APCH_ON 1 .
  • the selector 640 selects the comparison signal Vh output from the target voltage output unit 636 in response to the input of the APC mode signal APCH_ON 1 , and inputs the comparison signal Vh to the comparator 641 .
  • the selector 650 connects the terminal 650 - 2 with the terminal 650 - 4 in response to the input of the APC mode signal APCH_ON 1 .
  • the bias current Ib is supplied to the light-emitting point 301 .
  • the transistor 644 can transmit electricity, and the switching current source 645 supplies the switching current Isw to the light-emitting point 301 . Since current is supplied in the state in which the bias current Ib is supplied, the light-emitting point 301 emits laser light. The laser light emitted from the light-emitting point 301 is incident on the PD 204 . The PD 204 outputs a detection signal corresponding to the light power of the laser light.
  • the comparator 641 compares the comparison signal Vh from the selector 640 with the amplification signal Samp (Vamp) from the amplifier circuit 642 , and outputs a signal based on the comparison result to the selector 634 .
  • Vamp amplification signal
  • the comparator 641 causes the H hold capacitor 646 to discharge electricity. If discharge of the H hold capacitor 646 is continued, the light power of laser light incident on the PD 204 is decreased, and becomes close to the third light power Ph.
  • the comparator 641 holds the voltage of the H hold capacitor 646 in this state.
  • the light power of laser light emitted from the light-emitting point 301 and being incident on the PD 204 is controlled to be the third light power Ph by controlling the voltage of the H hold capacitor 646 .
  • the voltage regulating circuit 653 is connected between the H hold capacitor 646 and the switching current source 645 .
  • the voltage regulating circuit 653 receives a voltage control signal (not shown) from the CPU 401 .
  • the voltage control signal is a signal for regulating the voltage of the H hold capacitor 646 .
  • the CPU 401 generates the voltage control signal based on the state of the image forming apparatus (for example, sensitivity of photosensitive drum with respect to laser light, charging state of toner, and temperature in apparatus) and based on the environmental state in which the image forming apparatus is installed (temperature, humidity).
  • the switching current source 645 supplies the switching current Isw with the value based on the voltage regulated by the voltage regulating circuit 653 , to the light-emitting point 301 .
  • the voltage regulating circuit 653 also receives the APC mode signal APCH_ON and the VDO mode signal. If the APCH_ON signal is input, the voltage regulating circuit 653 does not regulate the voltage of the H hold capacitor 646 according to the voltage control signal.
  • second light power ⁇ first light power ⁇ third light power is established.
  • the magnitudes of the light powers are not limited thereto.
  • the LVDS receiver 601 outputs the PWM signal to the AND circuit 652 .
  • the PWM signal is input from the LVDS receiver 601 to one terminal of the AND circuit 652 , and a mode signal (VDO mode signal) is input from the mode channel decoder 633 to the other terminal of the AND circuit 652 . If the VDO mode signal input to the AND circuit 652 is at H level and if the PWM signal input to the AND circuit 652 is at H level, the AND circuit 652 outputs a signal at H level. If at least one of the VDO mode signal and the PWM signal input to the AND circuit 652 is at L level, the AND circuit 652 outputs a signal at L level.
  • the output signal from the AND circuit 652 is input to one terminal of the OR circuit 643 , and the APCH_ON signal from the mode channel decoder 633 is input to the other terminal of the OR circuit. If at least one of the output signal from the AND circuit 652 and the APCH_ON signal is at H level, the OR circuit 643 outputs a signal at H level. If both the output signal from the AND circuit 652 and the APCH_ON signal are at L level, the OR circuit 643 outputs a signal at L level.
  • the output of the OR circuit 643 is connected with a base terminal of the transistor 644 .
  • a collector terminal of the transistor 644 is connected with the light-emitting point 301 .
  • an emitter terminal of the transistor 644 is connected with the switching current source 645 .
  • the switching current source 645 leads the switching current Isw from VCC. Accordingly, the switching current Isw is supplied to the light-emitting point 301 for emitting laser light. If the signal at L level is output from the OR circuit 643 , the area between the collector terminal and the emitter terminal of the transistor 644 becomes a current non-conducting state.
  • APC sequence that is a feature of the image forming apparatus of this embodiment is described. Execution timings of the first light power control, second light power control, and third light power control according to APC on the each light-emitting point are controlled by the APC mode signal group (APC mode signal APCH_ON, APCM_ON, APCL_ON) output from the mode channel decoder 633 .
  • APC mode signal APCH_ON, APCM_ON, APCL_ON the APC mode signal group output from the mode channel decoder 633 .
  • FIG. 7A is a table showing the mode select signals, the channel select signals, and the IC select signal output from the CPU and corresponding to various control modes.
  • DIS represents DIS mode
  • ACC represents ACC mode
  • VDO represents VDO mode
  • OFF represents OFF mode
  • APCH “APCM,” and “APCL” respectively represent the third light power control, the first light power control, and the second light power control.
  • Wording “ic” represents an IC select signal. If the input mode select signal indicates execution of APC and the IC select signal is at L level, the laser drivers 405 A and 405 B are brought into a state in which the laser drivers 405 A and 405 B can execute the first light power control, the second light power control, and the third light power control.
  • Each control mode is controlled based on a combination of the mode select signals ms 0 , ms 1 , ms 2 , and ms 3 shown in FIG. 7A .
  • [1] in the table indicates all combinations other than a combination of mode select signals in any of DIS mode, ACC mode, APCH mode, APCM mode, and APCL mode.
  • [2] in the table represents “don't care” and represents that the control state is determined regardless of a pd control signal and the channel select signal (ch 0 , ch 1 , ch 2 , ch 3 ).
  • [*] in the table indicates a combination of channel select signals shown in FIG. 7B . Characters e 1 to e 16 in FIG. 7B respectively correspond to the light-emitting points 301 to 316 .
  • the first light power control is executed on the light-emitting point 305 .
  • the mode channel decoder 633 controls only APCM_ON 5 at H level among the 48 APC mode signals, based on the mode-select signals and the channel select signals, and controls the other APC mode signals at L level.
  • FIGS. 8A to 8D are illustrations explaining execution orders of the first light power control, second light power control, and third light power control for the light-emitting points 301 to 332 in the APC mode in the image forming apparatus according to this embodiment.
  • FIG. 8A to FIG. 8D are examples of execution orders of the first light power control, second light power control, and third light power control.
  • Data relating to an execution order is stored in the EEPROM 410 so that the first light power control, second light power control, and third light power control are executed in any one of the orders when the image forming apparatus is assembled.
  • the mode channel decoder 633 outputs the APC mode signal by using the table shown in FIGS. 7A and 7B so that the first light power control, second light power control, and third light power control are executed in the order.
  • the row number indicates the scanning period
  • the column number indicates the order of the light power control for each light-emitting point in each scanning period.
  • sign H in one cell in the table indicates the third light power control
  • M indicates the first light power control
  • L indicates the second light power control.
  • a number attached to each of H, M, and L indicates a light-emitting point on which the light power control is executed.
  • H 1 indicates that the third light power control is executed on the light-emitting point 301
  • M 4 indicates that the first light power control is executed on the light-emitting point 304 .
  • FIG. 8A indicates a sequence that completes APC on the light-emitting points 301 to 332 in 12 scanning periods. If the APC sequence shown in FIG. 8A is set in the EEPROM 410 , the CPU 401 executes the third light power control on the light-emitting points 301 to 308 in the N scanning period, executes the first light power control on the light-emitting points 301 to 308 in the next N+1 scanning period, and executes the second light power control on the light-emitting points 301 to 308 in the N+2 scanning period.
  • the image forming apparatus of this embodiment executes different light power controls on the same light-emitting point in a plurality of scanning periods. That is, the laser driver 405 A executes the first light power control on a certain light-emitting point group in a period from generation of a first BD signal (first synchronization signal) to generation of a second BD signal (second synchronization signal). Then, the laser driver 405 A executes the second light power control on the light-emitting point group in a period from generation of the second BD signal to generation of a next third BD signal (third synchronization signal).
  • the laser driver 405 A executes the third light power control on the light-emitting point group in a period from generation of the third BD signal to generation of a fourth BD signal (fourth synchronization signal).
  • the execution order of the first light power control, second light power control, and third light power control is not limited thereto.
  • the CPU 401 executes the third light power control on the light-emitting points 309 to 316 in the N+3 scanning period, executes the first light power control on the light-emitting points 309 to 316 in the next N+4 scanning period, and executes the second light power control on the light-emitting points 309 to 316 in the N+5 scanning period.
  • the CPU 401 executes the third light power control on the light-emitting points 317 to 324 in the N+6 scanning period, executes the first light power control on the light-emitting points 317 to 324 in the next N+7 scanning period, and executes the second light power control on the light-emitting points 317 to 324 in the N+8 scanning period.
  • the CPU 401 executes the third light power control on the light-emitting points 325 to 332 in the N+9 scanning period, executes the first light power control on the light-emitting points 325 to 332 in the next N+10 scanning period, and executes the second light power control on the light-emitting points 325 to 332 in the N+11 scanning period. After the second light power control in the N+11 scanning period is ended, the CPU 401 returns to the APC sequence in the N scanning period again. As described above, the CPU 401 executes APC on the respective light-emitting points over the plurality of scanning periods.
  • the Ib calculation unit 649 calculates the value of bias current Ib based on the control result of the first light power control executed in the N+1 scanning period and the control result of the second light power control executed in the N+2 scanning period. Also, when the second light power control on the light-emitting points in the N+2 scanning period is completed, the Ib calculation unit 649 calculates the value of bias current Ib based on the control result of the first light power control executed in the N+1 scanning period and the control result of the second light power control executed on the N+2 scanning period. That is, the Ib calculation unit 649 calculates the value of bias current Ib based on the latest voltage of the M hold capacitor 647 and the latest voltage of the L hold capacitor 648 .
  • FIG. 8B shows an example in which the respective light power controls according to APC are executed on four light-emitting points per one scanning period, and the respective light power controls according to APC are completed once in 24 scanning periods.
  • FIG. 8C shows an example in which the respective light power controls according to APC are executed on three light-emitting points per one scanning period, and the respective light power controls according to APC are completed once in 36 scanning periods.
  • FIG. 8D shows an example in which the respective light power controls according to APC are executed on two light-emitting points per one scanning period, and the respective light power controls according to APC are completed once in 48 scanning periods.
  • the APC sequence of the image forming apparatus is set so that light power control is continuously executed on at least two light-emitting points in which the same light power is set as a target light power as shown in FIG. 8A to FIG. 8D .
  • the CPU 401 executes the third light power control on the light-emitting points 301 to 308 in the N+1 scanning period.
  • the CPU 401 executes the first light power control on the light-emitting points 301 to 308 in the N+2 scanning period.
  • the CPU 401 executes the second light power control on the light-emitting points 301 to 308 in the N+2 scanning period.
  • FIG. 9 is a timing chart showing the N scanning period in the image forming apparatus in which the APC sequence shown in FIG. 8B is set. It is assumed that one scanning period is 500 ⁇ sec.
  • the CPU 401 controls the light-emitting point 301 in ACC mode (ACC 1 : 50 ⁇ sec) so that laser light from the light-emitting point 301 is incident on the BD 210 .
  • ACC mode ACC 1 : 50 ⁇ sec
  • BDn BD signal
  • the CPU 401 controls the light-emitting points 301 to 316 in OFF mode (25 ⁇ sec), and then controls the light-emitting points 301 to 316 in VDO mode (300 ⁇ sec).
  • VDO mode the CPU 401 controls the light-emitting points 301 to 316 in OFF mode (50 ⁇ sec).
  • the CPU 401 provides the third light power control on the light-emitting point 301 , light-emitting point 302 , light-emitting point 304 , and light-emitting point 303 in that order.
  • the mode channel decoder 633 outputs the APC mode signals which become at H level in the order of APCH_ON 1 , APCH_ON 2 , APCH_ON 4 , and APCH_ON 3 based on the mode select signal and the channel select signal output from the CPU 401 shown in FIG. 9 .
  • the output time of APCH_ON 1 is longer than the output times of the APCH_ON 2 , APCH_ON 4 , and APCH_ON 3 . This is because, since the third light power control on the light-emitting point 301 is executed first in the series of light power controls, the time, in which the output of the PD 204 receiving laser light from the light-emitting point 301 is unstable, is relatively long. With regard to that the time in which the output from the PD 204 is unstable is relatively long, the image forming apparatus of this embodiment is designed so that the time of light power control which is executed first on a light-emitting point in the series of light power controls is longer than the time of the light power control which is executed next on a light-emitting point.
  • the output time of APCH_ON 1 is set at 20 ⁇ sec, and the output time of APCH_ON 2 , APCH_ON 4 , and APCH_ON 3 is set at 9 ⁇ sec.
  • the output time of the APC mode signal is set so that the series of light power controls is ended in 50 ⁇ sec.
  • the CPU 401 After execution of APC, the CPU 401 generates a BD signal BDn+1 by controlling the light-emitting points 301 to 316 to OFF mode (25 ⁇ sec) and then controlling the light-emitting point 301 in ACC mode again.
  • FIGS. 10A to 10C are illustrations each showing a change with time of the output signal of the PD 204 in the APC sequence shown in FIG. 8B .
  • the vertical axis in any of FIGS. 10A to 10C plots the output (mV) of the PD 204
  • the horizontal axis plots the time ( ⁇ sec).
  • FIG. 10A shows a change with time of the output signal of the PD 204 when the third light power control is executed on the light-emitting points 301 to 304 in the N scanning period.
  • FIG. 10B shows a change with time of the output signal of the PD 204 when the first light power control is executed on the light-emitting points 301 to 304 in the N+1 scanning period.
  • FIG. 10 C shows a change with time of the output signal of the PD 204 when the second light power control is executed on the light-emitting points 301 to 304 in the N+2 scanning period.
  • the image forming apparatus of this embodiment can reduce the time required until completion of the light power control per one-time light power control to 9 ⁇ s.
  • the four-time light power controls with the same target light power can be executed within the APC execution period of 50 ⁇ s available for one-time scanning in the image forming apparatus of this embodiment as shown in the following expression. 20 ⁇ sec(light power control time of light source 301)+ ⁇ 0.1 ⁇ s(light-off period)+9 ⁇ s(light power control of light sources 302 to 304) ⁇ 3 ⁇ 50 ⁇ sec Expression (1)
  • FIGS. 11A and 11B each show a time required for APC execution in an APC sequence in which light power control with the same target light power is not continuously executed.
  • FIG. 11A shows a detection signal of a PD in an APC sequence in which light power control on the light source 302 is started after light power control on the light source 301 is completed and before the output of the PD is converged to zero.
  • FIG. 11B shows a detection signal of the PD in an APC sequence in which light power control on the light source 302 is started after light power control on the light source 301 is completed and after the output of the PD is converged to zero.
  • the APC sequence in the image forming apparatus of this embodiment is not limited to the patterns in FIG. 8A to FIG. 8D , and may be other sequence pattern as long as the pattern has a sequence in which light power control with the same target light power is continuously executed.
  • the first light power control may be continuously executed on the light-emitting points 301 and 302
  • the second light power control may be executed on the light-emitting points 303 and 304 in the N scanning period.
  • the APC sequence is desirably an optimal APC sequence based on the configuration of the optical scanning device.
  • an image forming apparatus in which, in a period other than the period of scanning on the photosensitive drum within one scanning period, laser light reflected by a polygonal mirror in a certain rotation phase is reflected by an inner wall of the optical scanning device and reaches the photosensitive drum.
  • the execution time of APC has to be short in a period other than the period of scanning on the photosensitive drum within one scanning period, a designer sets an APC sequence with a small number of light-emitting points on which the light power control of APC is executed within one scanning period as shown in FIG. 8D .
  • the image forming apparatus of this embodiment decreases the time required for the light power control by continuously executing the light power control with the same target light power on at least two light-emitting points. Accordingly, the number of light-emitting points on which the light power control can be executed within one scanning period can be increased as compared with the image forming apparatus that continuously executes light power control with different target light powers. A decrease in frequency of execution of APC on respective light-emitting points can be restricted.
  • the image forming apparatus that forms an image on the photosensitive member by using the light beams emitted from the plurality of light-emitting points, by continuously executing the light power control with the same target light power on different light-emitting points, a decrease in frequency of execution of the light power control on the plurality of light-emitting points can be restricted.

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